Summary: Background: Human brain continues to develop during the first few years of postnatal life. Since the developing brain is uniquely sensitive to damage following virus infection, administration of neurovirulent vaccines to infants can place the child's nervous system at increased risk for vaccine related injury. There is a need to develop a test to assess the vaccine's human neurovirulence potential, in order to develop the safest vaccines possible. Such an effective test currently does not exist for most new vaccines. Vaccine neurovirulence concerns have been raised for mumps virus, vaccinia virus, influenza virus, human parainfluenza virus III, poliovirus, dengue virus, Japanese encephalitis virus and human immunodeficiency virus. Thus, neurovirulence safety tests (NVST) are needed to identify a multitude of potentially neurovirulent vaccines that may cause CNS disease following vaccination. Progress: 1. Mumps vaccine: A newborn rat neurovirulence test was developed and neuropathological abnormalities were present. The most prominent abnormality was hydrocephalus. All strains induced some degree of hydrocephalus in rat brain and the severity of hydrocephalus tracked with the known human clinical histories of the virus strains. Using hydrocephalus as a marker of neurovirulence, wild-type strains could be distinguished from vaccine strains and the relative neurovirulence among wild type strains and among vaccine strains could be distinguished as well. These experiments in other laboratories outside of CBER are underway to initiate early stages of validation of this model system. In addition, virus genome correlates of neurovirulence are being investigated. 2. Influenza vaccine: Although rare, CNS events associated with wild type influenza virus can occur. In this project we are developing an assay to evaluate the relative neurovirulence of influenza viruses (wild type and vaccine) using our newborn rat model. Preliminary results suggest we can differentiate between wild type influenza virus and vaccine strains in our neurovirulence test. 3. Vaccinia-derived vaccines: We have developed a newborn mouse model for differentiating the virulence of vaccinia virus strains. Preliminary results suggest we can differentiate between vaccinia strains of greater or lesser virulence, suggesting this test will have utility as a pre-clinical virulence test.